The fact remains you know that demons wish to open up the fade then the chances are that as soon as they cross they may pull something like the breach. humanmade or demon made is irrelevant. you know a group have the knowledge ,power and power to pull this of. perhaps another group entered the fray.
Simply put the chantry and the templars knew and didnt do anything.
I live in the netherlands and after 1953 flood we said **** this **** and make sure nothing was left to chance. And nothing like the breach happened? Dude the circle of magi quest and wardens keep in DAO was basically the miniature version of what happened in DAI. yet not once people said : hey lets figure out how that veil works and we can do to keep it shut.
And mages where restricted what they where allowed to research.
You can't keep The Veil shut. It's impossible. And again, it's not a natural event. Demons can't force it to open from The Fade, so they aren't The one doing it (and in the case they would, you Couldn't stop them).
Again, how do you prevent someone to do it? You can't control every person in Thedas. You can reinforce The Veil, but that's after it's broken, not before. And that works only for small holes, not for Something as big as The Breach...which you can't experiment how to close without breaking it. So there was anyway to know how to solve this mess before if happened.
Also, mages can research The Fade, as they do. We do know how The Veil works, and numerous people studied it.
This event was impossible to prevent. Its scale is too big to be Fixed by known means. The only way to learn is by actual experience.